Results to date from this project have shown that (1) selection for large or small litter size, increased ovulation rate, and increased embryo survival leads to substantial direct response in each case; and (2) the resulting strains differ in a series of correlated traits, including regularity and duration of estrous cycles, circulating levels of FSH, PRL and progesterone, and sensitivity to exogenous hormone treatments. Genetic analysis of certain of the strains has established that genes affecting ovulation rate tend to act additively, whereas genes responsible for variation between dams in maternal performance show a strong degree of dominance for high survival. The primary objectives for the coming year are to characterize additional strains with regard to circulating levels of prolactin, progesterone and estrogen, to obtain additional information on target organ sensitivity to these hormones, to measure certain reproductive traits in males of these strains, and to utilize this information in an attempt to identify more specifically the underlying endocrine mechanisms responsible for the genetic changes resulting foom selection. From this approach it should be possible to obtain a more precise and more quantitative understanding of factors responsible for variation in mammalian reproductive performance. BIBLIOGRAPHIC REFERENCE: Michael, Sandra D., I. I. Geschwind, G. E. Bradford and G. H. Stabenfeldt. 1975. Pregnancy in mice selected for small litter size: reproductive hormone levels and effect of exogenous hormones. Biol. Reprod. 12:400-407.